1. Field of the Invention
This invention relates generally to components for reducing electromagnetic interference and, more particularly to an integrated filter featuring differential-mode and common-mode noise filtering capabilities.
2. Description of the Related Art
Electromagnetic interference (EMI) or electrical noise disrupts the normal operation of electronic equipment such as power supplies, electromechanical devices and computer systems. For a computer system, such an electromagnetic disruption may be caused by its external cabling, power supplies, electronic components, signal transmission paths, printed circuit boards and the like. Such a disruption may even be caused by its external environment because an electronic device that radiates electrical noise is also susceptible to electrical noises from other sources. In addition, the Federal Communications Commission (FCC) had imposed legal limits on the strength of radiated and conducted EMI that may be generated by devices that use timing circuits operating at frequencies above 9 kilohertz. Therefore, when designing an electronic equipment, it is desirable for EMI or electrical noise be kept at a minimum.
Electrical noise propagates either via air by electromagnetic fields or via conducting paths by currents. When electrical noise propagates via the conducting paths, there are two modal paths by which it travels, namely, the common-mode and the differential-mode. In general, the distinction between the two noise modes could be understood as follows: When a pair of parallel conductors connects two electronic circuit blocks together, the conductors would typically carry current signals. Each of the two current signals could be viewed equivalently as having two current components, a differential-mode current component and a common-mode current component. The two differential-mode current components in the pair of conductors are oppositely directed and typically contain the desirable portion of the propagating signal. On the other hand, the two common-mode current components on the conductors are directed in the same direction. And, these common-mode currents typically contain the undesirable portion of the propagating signal; but, unfortunately, in practice, they are always present.
In order to reduce common-mode currents, conductors are typically wound around a ferromagnetic core such as a ferrite toroid inductor. The electromagnetic fluxes resulting from the combination of the common-mode currents and the ferromagnetic core create an impedance that in turn reduces the amplitude of the common-mode currents. However, the conductor windings around the ferromagnetic core has no effect on the oppositely directed differential-mode currents because the resulting electromagnetic fluxes would have balanced each other out.
Since differential-mode currents often contain the desirable current signals, one then reduces the differential-mode electrical noise not by reducing overall current amplitudes but by eliminating the higher frequency harmonics of the current components and leaving the fundamental or the operating frequency element undisturbed. Ferromagnetic beads such as ferrite beads, typically of a cylindrical shape each having a co-axial bore, when used, will selectively attenuate (depending on the ferrite material selected) the high frequency components without affecting the lower frequency components of the signal. In practice, ferrite beads are placed in series with the signal conductors to suppress the differential-mode noise.
A typical EMI filter includes an input and output series transformers and a shunt transformer coupled to ground, this transformer combination being configured so that the overall effect is the attenuation of the common-mode electrical noise. The common-mode signal reacts to a high input impedance followed by a low impedance to ground, followed by a high impedance on the output side of the filter. The high impedance on the output side of the filtering component forms a voltage divider with the common-mode impedance on the wiring thus reducing the amplitude of the common-mode signal on the wiring. In contrast, according to this configuration, the differential-mode electrical noise is virtually unimpeded, reacting to virtually no input or output impedance contributed by the filter itself.
In another typical EMI filter, a generally rectangular magnetic core is configured to be mounted onto a printed circuit board by an automated insertion system. This component has pairs of throughholes and a number of U-shaped electrical conductors extending therethrough. Some of the U-shaped conductors in a throughhole share a common air space for impeding common-mode noise. Others, for the purpose of attenuating differential-mode noise, do not share any common air space with any other conductors in the throughholes.
Generally, such a filter configuration is limited in its EMI filtering effectiveness. Filters as such being compatible with automated insertion systems for mounting onto printed circuit boards typically do not provide optimal magnetic core depth for adequate EMI filtering. For example, for printed circuit board mounting purposes, magnetic core depth is fixed throughout the core, if the core depth is optimized for common-mode noise filtering, then regardless of what the differential-mode noise filtering requirements may be, one is forced to accept the already stipulated core depth. This filter which by itself is susceptible to external EMI noises, further loses its EMI filter effectiveness by being mounted on a printed circuit board which is often the high gradient area of an EMI noise environment such as the inside of a high speed personal computer enclosure. Furthermore, this filter's performance in its common-mode noise filtering is degraded further still because the U-shaped conductors extending through this filter are neither tightly couple or physically twisted together but placed in a spaced relation as required by the automated insertion systems.
Therefore, it is desirable to provide an integrated and economical electronic component that features effective common-mode as well as differential-mode EMI noise filtering.